936
Ind. Eng. Chem. Res. 1998, 37, 936-943
MATERIALS AND INTERFACES Aqueous Subphase pH Influence on Nonionizable Material Monolayers. Structural and Rheological Characteristics Manuela Ruiz Domı´nguez, Isabel Gonza´ lez Narva´ ez, and Juan M. Rodrı´guez Patino* Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, Universidad de Sevilla, c/.Prof. Garcı´a Gonza´ lez, s/nu´ m., 41012 Sevilla, Spain
The interfacial behavior of products used as food emulsifiers depends on the aqueous-phase composition. In this work, we studied the subphase pH influence on structure, stability, and rheological parameters for monostearin monolayers spread on the air-water interface. An expansion was observed in the film structure for the acid aqueous phase, which was higher when pH was reduced. Monolayers are even more expanded when the aqueous phase is alkaline, but the expansion decreased as the pH increased. These effects may be due to changes in water dipole distribution around monostearin polar groups and, therefore, in the hydrogen-bond number between water molecules and between these and monostearin headgroups. The films were essentially elastic. The surface rheological properties depended on the monostearin concentration at the interface, but were not very frequency-dependent. The stability or the rheology of the films did not change significantly because of variations in the pH subphase. Introduction In recent years, aqueous systems containing polymers and surfactants have received increasing attention because of their importance in a number of industrial applications including cosmetics, pharmaceuticals, detergents, paints, and foods. The behavior of fluid-fluid interfaces is also important for different processes such as absorption, distillation, extraction, flotation, heterogeneous catalysis, etc. Because surfactant molecule interactions in the monomolecular interfacial layer (monolayer) play a role in the macroscopic properties of the film, control of the structural characteristics and stability of the monolayer is of practical importance in order to get the optimum functionalization. Previous studies from our laboratory have demonstrated that the formation, structural characteristics, and stability of food emulsifiers (monoglycerides, diglycerides, and proteins) depend on the interfacial and subphase compositions and the environmental conditions (temperature) as well (de la Fuente Feria and Rodrı´guez Patino, 1994, 1995a,b, 1996; Rodrı´guez Patino et al., 1992, 1993; Rodrı´guez Patino and Ruiz Domı´nguez, 1993, 1996). These results are of practical importance since food colloids contain mixtures of different emulsifiers in complex formulations including sugars, alcohol, salts, etc., and are processed, stored, and consumed under different temperature conditions. Food emulsions and foams are often submitted to pH variations in the manufacturing, and even during their storage (Dickinson and Stainsby, 1982). So it is inter* To whom all correspondence should be addressed. Telephone: +34 5 4557183. Fax: +34 5 4557134. E-mail: jmrodri@ cica.es.
esting to know the subphase pH effects on emulsifier behavior. The pH influence is also fundamental in the study of phospholipid films, which make up biological membrane constituents (Gorwyn and Barnes, 1990). There are many studies about the structural characteristics and stability with monolayers of ionizable materials, such as fatty acids, alcohols, or amines (Adamson, 1990; Binks, 1991; Sharma and Rakshit, 1990). In these cases, the ionic groups in film molecules are responsible for the differences observed. For these systems, equations have been established determining the pH values at the surface in relation to the pH in the bulk subphase (Gorwyn and Barnes, 1990). With weakly ionizable products, however, changes in the subphase pH may not be expected to have any influence on monolayer characteristics. In this work we complement previous studies by investigating the effect of subphase pH on the monolayer characteristics of an accepted emulsifier in food formulations (monostearin). We have carried out experiments with different devices (Langmuir- and Wilhelmy-type film balances and tensiometry) to characterize the monolayer under dynamic conditions and at equilibrium. Surface rheological properties were determined under dilational conditions. The results show differences in the structural characteristics as a function of the subphase pH. These differences will be explained by means of changes in the monolayer-monolayer and monolayer-subphase interaction balance. Moreover, the existence of interactions between film-forming components has consequences on the thermodynamic parameter values associated with the transition between liquid phases and on the surface rheological behavior.
S0888-5885(97)00574-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/05/1998
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 937
Materials and Methods Synthetic 1-monooctadecanoyl-rac-glycerol (monostearin) was supplied by Sigma with over 99% of purity. To form the surface film, monostearin was spread in the form of a solution, using hexane:ethanol (9:1, v/v) as a spreading solvent. Analytical grade hexane (Merck, 99%) and ethanol (Merck, >99.8%) were used without further purification. The water used as the subphase was purified by means of a Millipore filtration device (Milli-Q). To adjust the subphase pH, buffer solutions were used. An acetic acid/sodium acetate aqueous solution (CH3COOH/CH3COONa) was used for pH ) 5 and 6, and a commercial buffer solution called trizma ((CH2OH)3CNH2/(CH2OH)3CNH3Cl) for pH ) 8 and 9. All these products were supplied by Sigma (>99.5%). Ionic strength was 0.01 M in all the experiments. Measurements of surface pressure (π) vs average area per monoglyceride molecule (A) were performed on a fully automated Langmuir-type film balance (Lauda). The method has been described previously (Rodrı´guez Patino et al., 1992, 1993; Rodrı´guez Patino and Ruiz Domı´nguez, 1993). The monostearin solutions were spread on the subphase by means of a micrometric syringe at the lowest temperature (5 °C). Aliquots of 250 µL (7.8 × 1016 molecules) were spread in each experiment. The experiments were carried out at temperatures ranging between 5 and 45 °C. The same precautions as those in previous works were taken to allow for the evaporation of the spreading solvent (15 min of waiting before beginning the isotherm registration) and for the choice of compression rate (0.062 nm2‚molecule-1‚min-1) to ensure reproducibility (Rodrı´guez Patino et al., 1993; Rodrı´guez Patino and Ruiz Domı´nguez, 1993). Some experiments were repeated (at least twice). In these cases, the mean deviation was within (0.1 mN/m for surface pressure and (0.005 nm2/ molecule for area. Equilibrium spreading pressures at the air-liquid interface (πe) were measured by the Wilhelmy plate method by using a roughened platinum plate attached to a Kru¨ss digital tensiometer K10. Prior to every measurement, the plate was briefly heated until glowing by holding it above a Bunsen burner. The vessel was cleaned using chromic sulfuric acid, boiled for a prolonged period in distilled water, and then flamed with a Bunsen burner before being used. The measurements were carried out in a circular, thermostated dish having a surface area of 63.6 cm2 at temperatures ranging from 10 to 30 °C. After the surface tension of the film-free subphase had been determined, equilibrium spreading pressures were obtained by spreading several 50 mL aliquots of monostearin solutions on the aqueous subphase. The reduction in surface tension (σ) was continuously recorded by a device connected to the tensiometer. Equilibrium was assumed when the pressure did not change by more than 0.1 mN/m in 30 min. If no pressure increase was observed upon addition of more monostearin molecules, the final pressure value was taken as πe ) σ0 - σe, where σe is the surface tension at equilibrium and σ0 is the subphase surface tension. Some experiments were repeated (at least four times). It was found that πe could be reproduced to ≈0.5 mN/m. To obtain surface rheological parametersssuch as surface dilational modulus, elastic and viscous components, and loss angle tangentsa Wilhelmy-type film balance (KSV 3000) was used. Surface pressure was
directly measured by means of a roughened platinum plate. A continuous compression-expansion of two barriers, at a determined frequency, causes a sinusoidal oscillation in the film. The percentage area change was 3% in all cases. The phase lag in the surface pressure change was used to determine the surface dilational modulus according to Lucassen and van den Tempel (1972). The surface dilational modulus E, which was derived from the small change in surface pressure π, resulting from a small change in surface area A, may be described by eq 1, where π ) σ0 - σ is the surface pressure and σ is the surface tension for the film:
E)
dπ dσ )d ln A d ln A
(1)
The dilational modulus E can be described by eq 2:
E ) |E|(cos θ + i sin θ)
(2)
The real part of the dilational modulus or storage component is the dilational elasticity Ed (eq 3), and the imaginary part of this quantity is the loss component (eq 4). Its contribution can be expressed in terms of a surface dilational viscosity, ηd, ω being the oscillation frequency. From both parameters, it is possible to define the loss angle tangent (eq 5). If the film is elastic, the loss angle tangent is zero.
Ed ) |E| cos θ
(3)
ηdω ) |E| sin θ
(4)
tan θ )
ηdω Ed
(5)
The surface rheological experiments were performed at least twice. The reproducibility of the results was over 5%. Surface measurements are very sensitive to the presence of impurities, so extreme care was taken to ensure that all materials and equipment used in this study were clean. All glassware in contact with the sample was previously cleaned in ammonium persulfate-sulfuric acid and rinsed in deionized water. The absence of surface active contaminants in the aqueous subphase and in the spreading solvent was checked by surface tension measurements. No lots of solvents with a different surface tension than that accepted in the literature were used. The absence of surface active contaminants in the spreading solvent was checked by measuring the surface tension of the aqueous solutions before and after evaporation of a quantity of hexane/ ethanol mixture without any monostearin spread on the interface. The effect of the hexane/ethanol spreading solvent on the structural characteristics of the monostearin monolayers was negligible as observed in previous experiments (Rodrı´guez Patino and Martı´n Martı´nez, 1994, Rodrı´guez Patino et al., 1992, 1993). Results and Discussion Film Structure and Stability under Dynamic Conditions. π-A isotherms, at some representative temperatures, for monostearin monolayers spread on aqueous subphase at different pH, are shown in Figure 1. Monostearin films have a similar structural polymorphism to that previously observed with the same
938 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 1. π-A isotherms for monostearin monolayers spread on the air-aqueous phase interface as a function of subphase pH and temperature. Table 1. Molecular Area, A (nm2/molecule), as a Function of Surface Pressures, π (mN/m), Aqueous Subphase pH, and Temperature T (°C)
pH
π ) 10
π ) 20
π ) 30
π ) 40
10
5 6 7 8 9 5 6 8 9 5 6 7 8 9
0.350 0.320 0.310 0.350 0.298 0.370 0.330 0.353 0.300 0.580 0.500 0.517 0.537 0.477
0.310 0.287 0.280 0.317 0.207 0.320 0.290 0.317 0.267 0.383 0.310 0.320 0.327 0.280
0.280 0.267 0.260 0.283 0.250 0.283 0.267 0.277 0.247 0.317 0.263 0.253 0.280 0.237
0.260 0.247 0.230 0.263 0.237 0.260 0.247 0.257 0.233 0.267 0.237 0.230 0.250 0.220
25
40
lipid spread on water and other aqueous subphases (Rodrı´guez Patino et al., 1992, 1993; Rodrı´guez Patino and Ruiz Domı´nguez, 1993, 1996). That is, solid (S), liquid-condensed (LC), or liquid-expanded (LE) structures could exist, depending on the surface pressure, pH, and temperature. High temperatures produce more expanded structures, while an increase in the surface pressure condenses the monolayer toward the solid state. However, the pH dependence on the structural characteristics of monostearin films is more complex. Monostearin films spread on acid aqueous subphases (pH ) 5 or 6) are more expanded than those spread on pure water (pH ) 6.8). Moreover, the monolayer expansion is higher at lower pH values, that is, when the H+ concentration in the aqueous medium increases. As the aqueous subphase pH is higher than 7, the isotherms appear in higher molecular areas as well, but in this case, the expansion decreases as the OHconcentration increases. The extension of film expansion can be deduced better from molecular area values at several pH, temperatures, and surface pressures (Table 1). The area occupied by monostearin molecules decreases when the pH rises from 5 to 7, whatever the pressure or temperature is. When the subphase pH
Figure 2. Phase diagram for monostearin monolayers spread on aqueous solutions as a function of temperature and pH: (A) pH ) 5, (B) pH ) 6, (C) pH ) 8, and (D) pH ) 9. Deionized water subphase is included as reference (discontinuous line, Rodrı´guez Patino et al., 1992).
increases to 8, higher molecular areas are obtaineds similar to those observed at pH ) 5swhile more alkaline aqueous subphases lead to a decrease in molecular area, similar to that observed in the acid region. This behavior is similar to that observed with monolayers of some phospholipids (Gorwyn and Barnes, 1990). These lipids show a more expanded structure when the subphase pH increases from acid to alkaline values because of film molecule dissociation. Temperature and pH influence on monostearin structure can be analyzed in a phase diagram obtained directly from the π-A isotherms. Figure 2 shows phase diagrams for monostearin films at different pH values. The lines in these diagrams represent transitions between two structures of the monolayer or transitions toward collapse. The collapse pressure πc represents the highest pressure at which amphiphilic molecules can exist in a monolayer state. The area between the transition lines represents the respective zone for each structure. It can be seen that the area for the solid structure decreases at pH values lower than 7. This effect is caused by an increase in the pressure for the transition from liquid-condensed to solid structure and, especially, as a consequence of a reduction in the collapse pressure. So, the aqueous subphase acidity is favorable to monolayers with a liquid-condensed structure. Changes in the subphase pH do not have a significant influence on the area for the liquid-expanded phase nor on the pressure for the transition toward a liquid-condensed state, but this phase requires higher temperatures than when the subphase is deionized water. In the alkaline region, pressure values for the LC f S transition do not change significantly with temperature or pH. The collapse pressure values are lower at pH ) 8, so the area for solid structure is reduced, while those for liquid-condensed and liquidexpanded phases do not change. Changes take place on πc values as a consequence of subphase pH variations. More expanded monolayers are generally associated with lower collapse pressures.
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 939
Figure 3. Collapse pressure vs subphase pH as a function of temperature. Table 2. Limiting Area (Alim, nm2/molecule) and Molecular Area at the Beginning of the Compression (A0, nm2/molecule), as a Function of Aqueous Subphase pH and Temperature T (°C)
pH ) 5
pH ) 6
water
pH ) 8
pH ) 9
0.267 0.267 0.267 0.287 0.348
0.347 0.343 0.350 0.332 0.350 0.378
0.290 0.277 0.273 0.280 0.283 0.287
0.360 0.360 0.370 0.730 0.730
0.500 0.500 0.520 0.850 0.850 0.850
0.450 0.450 0.420 0.740 0.750 0.760
Alim 5 10 20 30 40 45
0.347 0.336 0.347 0.347 0.395 0.427
0.313 0.313 0.313 0.307 0.312 0.333
5 10 20 30 40 45
0.483 0.500 0.520 0.800 0.850 0.850
0.450 0.430 0.430 0.750 0.750 0.750
A0
The collapse pressure of a monostearin monolayer on an acid subphase increases with pH until a maximum is attained at pH ) 7 (Figure 3). However, the pH dependence on πc in an alkaline subphase is more complex. The collapse pressure decreases at pH ) 8 and then increases at pH ) 9. The influence of temperature on the collapse pressure is less significant than that of the pH. The minimum collapse pressure does not depend on temperature at pH ) 5 and 8. However, it can be seen that at pH ) 6 and 9, πc decreases as temperature increases, a phenomenon opposite to that observed at pH ) 7. The surface film expansion agrees with the limiting area (Alim) and the molecular area at the beginning of the monolayer formation (A0) values. Alim represents the transversal molecular area in the condensed state and is obtained by extrapolating the straight part of the isotherm to π ) 0. A0 represents the maximum distance at which film molecules begin to interact. Table 2 shows the values for both parameters, Alim and A0, as a function of pH and temperature. An increase in temperature leads to a reduction in molecular interactions, so higher values for Alim and A0 are obtainedsAlim for monostearin monolayers spread on an aqueous solution at pH ) 9 is an exception. A sudden increase in A0 occurs at temperatures closer to 25 °C, which is associated with a film transition from the LC to LE structure. The pH effect is different for Alim and A0. The molecular area at the beginning of the compression (A0) is higher when a buffered solution is used as subphase either at pH < 7 or at pH > 7, but the effect decreases as the film structure is LE. A reduction in A0 and Alim is observed for both alkaline and acid subphases as the pH increases. Previous results indicate that subphase pH variations especially affect films with a liquid-condensed structure,
Figure 4. π-A isotherms for monostearin monolayers spread on the air-aqueous phase interface. Temperature (°C): (A) 20 and (B) 40. Subphase composition: 9, deionized water; 2, aqueous solution buffered to pH ) 7.
expanding the film structure as the aqueous medium acidity increases. It can be suggested that ions from the buffer solution are responsible for the expansion observed, since the addition of salts to the aqueous subphase causes a similar effect on monostearin monolayers (Rodrı´guez Patino and Ruiz Domı´nguez, 1996). However, no differences were found in monostearin film structure when it was spread on deionized water (pH ) 6.8) and on a buffered solution at pH ) 7 by means of K2HPO4/KH2PO4. Figure 4 shows π-A isotherms for monostearin monolayers spread on pure water, and on an aqueous buffered solution at pH ) 7, at two representative temperatures. Similar results were observed at different pH values and at various temperatures (data not shown). So no expansion can be established as a consequence of the presence of phosphate salts in the aqueous medium at the concentrations used in this work (0.01 M). In other esters, a molecular hydrolysis justifies changes in a π-A isotherm or in structural parameters, such as the pressure value for the transition between states. This is the case of poly(isobutylcyanoacrylate) films studied by Min˜ones et al. (1993). The hydrolysis of these molecules has two opposite effects, a repulsion between equally charged molecules that expands the film and an ion solvatation that implies their introduction in the aqueous subphase and the film condensation. If monostearin hydrolysis occurs, alcohol (glycerol) and fatty acid (stearic acid) could be produced. At alkaline pH, the hydrolysis rate is higher, so glycerol and fatty acid molecules could be even completely dissociated and solvated. Anions from fatty acid molecules could form salts with the positive ions present in the aqueous medium, and a dissolution of glycerol in the subphase could occur. Both effects could reduce the molecular area because of film instability. In previous works, it was established that monostearin films spread on deionized water were stable because no isotherm displacement to the pressure axis was observed as temperature or surface pressure increased (Rodrı´guez
940 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Table 3. Equilibrium Spreading Pressure (πe), in mN/m, as a Function of Temperature and Aqueous Subphase pH pH ) 5 pH )7 pH ) 8
10 °C
20 °C
30 °C
25.7
27.0 26.0 25.8
30.4
23.9
31.3
Table 4. Thermodynamic Parameters for Liquid-Condensed to Liquid-Expanded Transition for Monostearin Monolayers Spread on Aqueous Subphases as a Function of pH
pH 5
6
8
9
Figure 5. Continuous compression-expansion cycles for monostearin monolayers spread on the air-aqueous phase interface at (A) pH ) 6 and (B) pH ) 9. Temperature 20 °C. Symbols: (O) 1st compression, (2) 2nd compression, and (×) 2nd and 3rd compressions.
Patino et al., 1992). Figure 5 shows results from monostearin monolayers spread on two characteristic aqueous subphases at pH ) 6 and 9, when continuous film compression-expansion cycles were realized. From the absence of hysteresis, even at the highest temperatures and surface pressures, it can be established that the results are reproducible and monostearin monolayers may be expected to be stable in the experimental conditions used here. Structural Characteristics at Equilibrium. Optimum use of surfactants in technological processes is only possible through an understanding of their interfacial behavior, including the equilibrium and the dynamic properties of their films on the liquid/gas or liquid/liquid interfaces (Miller et al., 1991; Miller and Kretzshmar, 1991). The equilibrium spreading pressure (πe) is the maximum surface pressure to which a spread film may be compressed without the possibility of monolayer collapse (Gaines, 1966). So the πe value is of primary importance to the stability of a monolayer, since at higher surface pressures the monolayer is not in a state of thermodynamic equilibrium (Gaines, 1966; Phillips and Hauser, 1974; Smith and Berg, 1980). The magnitude of πe for the monostearin film depends on the temperature and subphase pH (Table 3). Higher πe values are obtained when temperature increases.
T (K)
AE (nm2/ molecule)
AC (nm2/ molecule)
δπt/δT (mN‚ m-1‚ K-1)
303 308 313 318 303 308 313 318 303 305 313 318 303 308 313 318
0.75 0.63 0.55 0.46 0.71 0.58 0.50 0.44 0.70 0.61 0.52 0.45 0.66 0.56 0.46 0.40
0.36 0.35 0.35 0.33 0.30 0.31 0.29 0.29 0.33 0.32 0.32 0.32 0.28 0.27 0.26 0.26
1.23
πt (mN/ m)
∆Ht (kJ‚ mol-1)
∆St (J‚ mol-1‚ K-1)
1.0 5.6 12.0 19.4 0.7 5.0 10.0 17.0 2.0 6.0 11.3 18.4 1.7 5.3 11.0 17.3
88.7 63.4 45.2 24.7 79.6 56.1 40.7 28.3 74.2 57.2 38.4 25.7 73.8 54.9 36.6 25.7
292.6 205.9 144.5 77.8 269.8 182.1 130.1 89.1 244.9 185.8 122.8 80.7 243.5 178.3 117.0 80.9
1.08
1.09
1.05
From these data, together with the phase diagram (Figure 2), it can be concluded that the monostearin structure at equilibrium is liquid-condensed and the solid phase is a metastable state. Since the spreading process is determined by the nature of the lipid-lipid and lipid-subphase interactions in the monolayer and in the bulk phase (Jalal et al., 1980; Rodrı´guez Patino and Martı´n Martı´nez, 1994), changes on πe values are coincident with changes observed in the film structure as a function of pH. From a practical point of view, films with liquid-condensed structure exhibit good rheological properties for emulsification (Krog et al., 1985). Thermodynamic Parameters. The latent heat ∆Ht of a two-dimensional first-order phase transition and the entropy change ∆St for an isothermal reversible process can be calculated from a modified ClausiusClapeyron equation (Ito et al., 1989; Phillips and Chapman, 1968; Rettig and Kuschel, 1990; Yue et al., 1976):
∆Ht δπ ) δT T(AE - AC)
(6)
∆St ) ∆Ht/T
(7)
where the molecular areas at the beginning and at the end of the transition at temperature T, AE, and AC, are associated with the molecular area in the liquidexpanded and liquid-condensed phases, respectively. πt is the pressure at which two-dimensional condensation occurs. These parameters were deduced from the π-A isotherm as previously reported (Rodrı´guez Patino et al., 1993) and are shown with values for ∆Ht and ∆St in Table 4. Figure 6 shows ∆Ht and ∆St obtained for the LC-LE monostearin monolayer transition as a function of temperature and pH. It can be seen that the liquidcondensed-liquid-expanded transition requires energy (∆Ht > 0) and involves an increase in the system
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 941
Figure 6. Enthalpy and entropy vs temperature for liquidcondensed-liquid-expanded transition in monostearin monolayers spread on the air-aqueous phase interface as a function of pH: ([) 5, (O) 6, (2) 8, (*) 9, and (9) water.
disorder (∆St > 0). The numerical values are not very high; therefore, from a thermodynamic point of view, it is not difficult for the phase change to take place. An increase in temperature leads to a lower increment in enthalpy and entropy. This fact is in agreement with a process that is endothermic and increased in disorder. In fact, at the highest temperatures, the film molecules have more energy, so the molecular agitation and the separation between each molecule increase. Therefore, monolayer-monolayer interactions are reduced and, as a consequence, ∆Ht and ∆St decrease. The presence of a buffer solution produces higher values for entropy and enthalpy, especially at pH ) 5, as the film expansion is higher. In alkaline subphases, differences in enthalpy and entropy are reduced. The monostearin monolayers with a liquid-expanded structure spread on buffer solutions are favorable to the existence of a large accumulation of water molecules at the interface. If this condition exists, compression involves either the removal of intervening water molecules or a rearrangement of the polar group as the film condenses. Similar results have been obtained with monostearin monolayers spread on aqueous solutions containing solutes that expand monostearin films (Rodrı´guez Patino et al., 1993; Rodrı´guez Patino and Ruiz Domı´nguez, 1996). Thermodynamic parameters associated with the LC-LE transition are of utility to support the existence of an expansion in the film structure as a consequence of interactions between film-film and film-subphase components. Surface Rheological Properties. The behavior of fluids in processes such as extraction, distillation, foaming, and emulsification has been proved to depend on the occurrence and magnitude of surface tension gradients (Bos et al., 1997; Murray and Dickinson, 1996; Lo et al., 1983). Such gradients may be due to spatial variations in temperature or concentration along the surface, or they may arise as a result of local compression or expansion of the surface. In a quantitative analysis of these phenomena, the response of the surface to local compression or expansion is described by the surface dilational modulus (Lucassen and van den Tempel, 1972). This modulus, in conjunction with liquid densities, viscosities, and the characteristic rate of deformation, determines the resistance to the creation of surface tension gradients as well as to the speed at
Figure 7. Superficial density dependence on surface dilational modulus (E, mN/m), surface dilational elasticity (Ed, mN/m), loss component (ηd‚ω, mN/m), and loss angle tangent (tan θ) for monostearin films spread on aqueous solutions buffered as a function of pH at 20 °C. Angular frequency: 20 mHz. Values for surface dilational modulus deduced directly from the π-A isotherms (4) have been included.
which such gradients disappear once the system is left to itself (Lucassen and van den Tempel, 1972). The higher the surface dilational elasticity is, the more rapidly these processes restore the uniformity of the surface tension. When values for the surface dilational modulus are known as a function of frequency, predictions can be made about the importance of the Marangoni effect on a given system. The surface rheology of emulsifiers at fluid-fluid interfaces is of interest due to its importance in relation to emulsion and foam stability, and because of its extreme sensitivity to structure and the nature of intermolecular interactions at the interface (Dickinson et al., 1990; Earnshaw and McLaughlin, 1992). The surface viscoelastic properties of monostearin films spread on the air-water interface have been studied as a function of pH subphase (Figure 7). It can be seen that (i) the values for the surface dilational modulus are very similar to those for the surface dilational elasticity at every subphase pH and (ii) the surface dilational viscosity and the loss angle tangent values are low and practically zero. As a consequence of this behavior, it can be established that the surface dilational characteristics of monostearin films are essentially elastic. The values for the surface dilational modulus at different film structures under dilational conditions are similar to those deduced directly from the π-A isotherms slope. These values have been included in Figure 7. So it can be concluded that the elasticity modulus, as deduced from the π-A experiments in a
942 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 8. Frequency evolution of the surface dilational modulus for monostearin monolayers spread on the air-aqueous phase interface at (2, 4) pH ) 5 and (9, 0) pH ) 8. Monostearin superficial density (molecule‚nm-2): 3.7 (closed symbols) and 7.6 (open symbols). Temperature: 20 °C.
Langmuir film balance, provides a good measurement of the superficial rheological characteristics of emulsifier films when the surface dilational viscosity is practically zero, which is the case with insoluble monostearin films. The more condensed the structure is, the higher the elasticity modulus of the film becomes until collapse is reached. From this point, there is a stabilization in E values with a higher superficial density of surfactant molecules. Measurements of surface tension as a function of monostearin superficial density (Rodrı´guez Nin˜o et al., 1996) suggest that there exists a critical concentration at which the interactions between monostearin molecules lead to a constant surface activity as the interface is saturated with lipid. By analogy with emulsifiers that form micelles in the bulk phase, a critical superficial concentration (CSC) from the surface tension-superficial density relationship has been defined (Rodrı´guez Nin˜o et al., 1996). The value for CSC is close to surface density at the collapse point. The subphase pH effect on surface rheological parameters is significant for monostearin monolayers spread on acid subphase, especially as the film collapses (Figure 7). Subphase pH values at which monostearin monolayers are more expanded reduce the surface dilational modulus with no influence on the loss angle tangent. More acid subphases may lead to a decrease in the adhesion forces between molecules at the interface, which is consistent with the reduction observed in E. However, the pH effect on the surface dilational modulus is lower than the surface density effect. Changes in oscillation frequency over a range of 1-100 mHzsat two representatives pH values and two superficial molecular densitiessdo not have any influence on surface rheological parameters (Figure 8). So neither a reordering of the molecules at the interface nor an exchange of molecules between the interface and the bulk phase during expansion-compression cycles occurs during the experience. This behaviorswhich agrees with an essentially elastic surface rheological characteristicsis typical of insoluble lipids as was observed previously from rheological measurements with monostearin monolayers using other techniques (Rodrı´guez Nin˜o et al., 1996), and from relaxation experiments at constant surface pressure on a Langmuir film balance (de la Fuente Feria and Rodrı´guez Patino, 1994, 1995). Conclusions Changes in subphase pH lead to variations in the interactions between film molecules and between these
and the subphase components. Introducing a higher concentration of ions in the aqueous phase modifies the hydrogen-bonding number that can be established between water molecules or with the monostearin polar group, and the water molecule distribution around film headgroups. Changes observed in the film structure have to be discussed separately for the acid and the alkaline region, with those of acid subphases being the effects of highest interest for food colloid formulations. When the aqueous medium is rich in protons, which have to be hydrated, monostearin molecules are separated to permit these hydrated ions to introduce themselves between their polar groups. At the same time, film molecules orientate their -OH groups in the most convenient form to interact with these ions. From the results of dilational surface rheology, it can be concluded that monostearin films are almost entirely elastic, so π-A isotherm measurements in a Langmuir film balance provide a good method to determine the elasticity modulus and its variations as a function of temperature or subphase composition. Since dilational strains are often dominant when the expansion or compression of interfaces is involved, the dilational rheological properties of surfactant films are considered to be of great importance in engineering applications involving fluid-fluid interfaces. In a great number of model systems, a high dynamic or elastic modulus correlates with increased emulsion and foam stability (Djabbarah and Wasan, 1985). Acknowledgment This research was supported by DGICYT through Grant PB94-1459. Notation A ) average area/monoglyceride molecule (nm2/molecule) AC ) molecular area at the end of the liquid-expandedliquid-condensed transition (nm2/molecule) AE ) molecular area at the beginning of the liquidexpanded-liquid-condensed transition (nm2/molecule) Alim ) limiting area or the transversal molecular area in the condensed state (nm2/molecule) A0 ) molecular area at the beginning of the monolayer formation (nm2/molecule) E ) surface dilational modulus (mN/m) Ed ) surface dilational elasticity (mN/m) LC ) monolayer liquid-condensed structure LE ) liquid-expanded structure S ) solid structure T ) temperature (K) Greek Symbols ∆Ht ) enthalpy variation (kJ/mol) associated with the monolayer transition between liquid states ∆St ) entropy variation (J‚mol-1‚K-1) due to monolayer transition between liquid states ηd ) surface dilational viscosity (mN‚s/m) π ) film surface pressure (mN/m) ) σ0 - σ πc ) film collapse pressure (mN/m) πe ) equilibrium spreading pressure (mN/m) ) σ0 - σe πt ) surface pressure for the liquid-expanded-liquidcondensed film transition (mN/m) θ ) phase angle (°) σ ) film surface tension (mN/m) σe ) film surface tension (mN/m) at equilibrium σ0 ) subphase surface tension (mN/m)
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 943 ω ) oscillation frequency (mHz)
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Received for review August 19, 1997 Revised manuscript received November 26, 1997 Accepted December 1, 1997 IE970574L
